diff --git a/scico/linop/xray/_xray.py b/scico/linop/xray/_xray.py
index 5ae6ea179..c0465b236 100644
--- a/scico/linop/xray/_xray.py
+++ b/scico/linop/xray/_xray.py
@@ -26,6 +26,10 @@ class XRayTransform(LinearOperator):
     """X-ray transform operator.
 
     Wrap an X-ray projector object in a SCICO :class:`LinearOperator`.
+    **Warning:**  Note that the only X-ray projector object currently
+    supported, :class:`.Parallel2dProjector`, is not a very accurate
+    approximation of the integral transform representing real projection
+    imaging, and is therefore not suitable for real imaging applications.
     """
 
     def __init__(self, projector):